We live in an age which – unlike Caravaggio's – requires transgression of its artists.

No surprise, then, that – like Byron – Caravaggio, as a sexually profligate dandy, extremist and proto-Romantic transgressor, finds himself a darling of artistic biography. He has been the subject of three major lives in just 12 years: the others are Helen Langdon's sturdy 1998 account and Peter Robb's maverick M from 2000.

Exhibition schedules experience Caravaggio-mania, one major show opening before another closes – as presently, on the 400th anniversary of the painter's death, the Scuderie del Quirinale's handsome survey in Rome gives way to three complementary ones in Florence. The relatively modest body of secure attributions race around the globe in the spirit of their progenitor. He left Milan for Rome, Rome for Naples, Naples for Malta, Malta for Sicily, and Sicily for Naples and Rome, weighed down by one proven murder, numerous attacks, physical and verbal, brawls, contestations and many pettier crimes.

As Andrew Graham-Dixon's Caravaggio hits the shelves, it was confidently announced in Tuscany's Porto Ercole – where the artist certainly died – that his remains had been identified. Studies of one set of bones produced a strong genetic match to others from the small Lombard village by whose name the artist would always be known.

Gaps in our knowledge have incited novelists to exhume him – in Norwegian, Italian and English. Derek Jarman's 1986 film gave us our own Caravaggio. Different cultures have placed divergent emphases on the naturalist master, with arguments for Caravaggio's homosexuality proliferating in Anglo-American accounts. Italian historians have concentrated on pursuing signs of his spirituality. It is, presumably, in acknowledgment of this bifurcated persona that Graham-Dixon's copious life offers us a Caravaggio both sacred and profane. Still, he cannot and does not resolve the many contradictions.

We have reams of uncontested documentation about Caravaggio's life, because of the frequency with which he was arrested, cautioned or subjected to legal challenge. Some incidents, however familiar, add vitality to Graham-Dixon's account of an artist secure in his genius, but petulant and superior. Take one taverna episode. Caravaggio asked which of a plate of artichokes had been cooked in butter and which in oil. The waiter replied that he need only smell them to know - which struck Caravaggio as impugning him. He was being treated as an outsider without taste; as a Lombard hick. Flinging the plate at the waiter, he saluted him thus: "Becco fottuto" ("fucked-over cuckold"). In Caravaggio's 38 years, scores were pronounced becci fottuti.

Graham-Dixon assiduously draws from the deductions of earlier scholars, particularly Langdon. On occasion, he finds new routes to investigate, and is not shy of making a few bold claims. The strongest gambit is that Caravaggio – aside from being the great artistic innovator, drunkard, criminal and insecure, vengeance-seeking lowlife – was a pimp. Sadly, the evidence here is simply unconvincing. Elsewhere, he proceeds more cautiously.

When Michelangelo Merisi was born in 1571, Italian art approached a sort of stasis, caused by the strangulating effects of Church proscriptions. Hindsight glosses the period's output as Mannerist, in the theatrical spirit of the other Michelangelo. Repeatedly, Caravaggio turned back to instances of his Florentine namesake's innovation, challenging them, stealing from them and surpassing them. His understanding of the Venetian Giorgione's use of shade may have grounded his tenebrism, but in most respects, Caravaggio's limited engagement with artistic example was answered by a single Florentine predecessor.

A fine example involves his workshop assistant Cecco adopting a pose identical to one of Michelangelo Buonarroti's ignudi. These musclebound nudes on the Sistine Chapel ceiling were transgressive enough, invoking a Classical golden age in a place of Christian worship. But Caravaggio's innovation is still more daring. Cecco, perhaps 12, is a radiant, recumbent boy John the Baptist, frolicking in the unguarded wilderness with a horned sheep. Graham-Dixon does not answer why Caravaggio reconceived the usual Lamb of God. But this was surely an act of intentional counterpoint. Our first impression – that this John is too immature to merit such sensual depiction - is shockingly confirmed by the inversion of pictorial convention. Instead of the adult John looking over the vulnerable lamb, Caravaggio has a mature sheep nuzzling the boy.

Caravaggio's assumed sexuality will remain contentious. Graham-Dixon errs towards Langdon's scepticism, even as innuendoes against the painter stack up. It is true that key sources are replete with jealousy and sleight-of-hand. Nowhere is this truer than in the case of Giovanni Baglione's 1642 biography.

The trouble with discounting charges of pederasty as commonplace slanders is that the history of Western homosexuality has always been a history of homophobia too. To dispose of accusations against Caravaggio simply because homophobic slurs were widespread may thus be to make an elementary mistake. Nobody – to our knowledge - called Caravaggio a becco fottuto. Had they done so, we might still consider his possible cuckoldry.

Nor is Howard Hibbard's 1983 argument for a boy-loving Caravaggio as readily dismissed as Graham-Dixon implies. Of "Boy with a Basket", he argues: "Those who subscribe to the romantic myth of Caravaggio as a social and sexual outsider, boldly expressing the love that dares not speak its name, are obliged to twist the fruit-bearer's expression of amorous yearning into the come-hither eyelash-flutterings of a rent boy." Mostly, Graham-Dixon treads with greater subtlety. But a critic who interprets Caravaggio's acquaintance with streetwalking women as evidence that he was a pimp had better not accuse others of loaded arguments.

A more forceful problem for anyone who rejects the notion of a homoerotic sensibility in Caravaggio's works remains in the cumulative insistence of the canvases – from the open-lipped "Lute Player" to the boy angel ministering to "St Francis in Ecstacy" and the adolescent angel provocatively thrust upon "The Rest on the Flight into Egypt".

This uneven book has much to commend it, particularly in what it gives us of contemporary sources, lifestyles and mores. It is, however, often disorderly – especially when, at the outset, Graham-Dixon abandons Caravaggio at his birth, only rediscovering his subject on page 44. Moreover, Graham-Dixon makes a couple of debatable adjustments to accepted dates. And closing your account of a 17th-century life with a two-page quotation from Martin Scorsese is simply too bizarre. An additional, chapter-length account of Caravaggio's influence (on Rembrandt, Rubens, the Romantics and the Moderns) would have done better, usefully taking in his impact on the cinema directors Pasolini - here accorded a single sentence - and Jarman, who is missing altogether.